ESXi is telling fibs to backup software

Embiggened VMs may not be backed up until you turn off CBT and turn it on again

Redditor “tottenham12712” has pieced together a scary scenario for VMware users: ESXi might be feeding dud data to backup software.

VMware knows it has a problem. Indeed, it has explained in this knowledge base article that when vAdmins expand a vmdk file past 128GB, and Change Block Tracking (CBT) is enabled, the QueryChangedDiskAreas("*") command returns an incorrect list of virtual machine disk sectors.

As tottenham explains, that's bad news because some backup software relies on that data to determine what to back up.

Tottenham12712 goes on to quote an email he's received from Veeam containing the following grim news:

“But the main point is that your backups and replicas for all VMs that had its virtual disk size expanded beyond 128 GB at some point may be unrecoverable.”

The good news is that there seems to be a simple fix: turning it off and turning it on again, with the “it” being Change Block Tracking.

VMware's working on a proper fix and Veeam is working on updates to its software to take the flaw into account. ®

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